1. Field of the Invention
This invention relates to an abnormality detecting apparatus and an abnormality detecting method for an air/fuel ratio sensor, which is capable of detecting an air/fuel ratio within a wide range that includes the stoichiometric air/fuel ratio.
2. Description of the Related Art
A typical internal combustion engine burns a mixture of air and fuel in a combustion chamber and discharges exhaust gas produced by that combustion to the outside through an exhaust passage. The exhaust passage is provided with an oxygen sensor or an air/fuel ratio sensor to detect the air/fuel ratio of the mixture from the oxygen concentration in the exhaust gas. In the internal combustion engine, feedback control is performed by adjusting the fuel quantity so that the air/fuel ratio detected by this air/fuel ratio sensor becomes a target air/fuel ratio (usually the stoichiometric air/fuel ratio) that is set in advance.
If an abnormality occurs in the air/fuel ratio sensor, however, this type of air/fuel ratio feedback control is no longer able to be appropriately performed, so various measures (such as switching to open loop control) are performed in response to a detected abnormality.
More specifically, abnormality detection for an air/fuel ratio sensor is performed by detecting the temperature of the sensor and the reciprocal of the sensor's resistance value, i.e., the admittance indicative of the ease with which current flows, and comparing that with a determining value.
As shown in the routine illustrated in the flowchart in FIG. 6, the abnormality detecting apparatus for this air/fuel ratio sensor detects the admittance of the sensor when a period of time T has passed after startup of the internal combustion engine (steps S210, S220, and S230). This period of time T is the time required to ensure output from the air/fuel ratio sensor after the engine is started, and is longer than a period of time SA required for the air/fuel ratio sensor to be heated to an activation temperature by a heater. This period of time T is obtained by testing or the like beforehand.
Next, an abnormality in the air/fuel ratio sensor is detected by comparing the detected admittance with a determining value YJ (step S240). Here, there is a tendency for the admittance at the same temperature to drop when there is a decline in performance of the air/fuel ratio sensor due to, for example, deterioration over time. Therefore, as shown in FIG. 7, a decline in performance of the sensor can be detected when the detected admittance is less than the determining value YJ.
When an abnormality is detected (i.e., YES in step S240), a warning lamp indicating an abnormality in the sensor is illuminated and the control system of the fuel quantity is switched to abnormality response control (steps S250 and S260).
When a disconnection abnormality has occurred in the air/fuel ratio sensor, current stops flowing to the sensor so the admittance of the sensor becomes “0”. Thus, in both the case in which there is a decline in performance of the sensor as well as the case in which there is a disconnection abnormality in the sensor, the admittance of the sensor becomes lower than the determining value YJ. Therefore, the abnormality detecting apparatus for the air/fuel ratio sensor detects these abnormalities by comparing the admittance with the determining value.
However, while it is necessary to determine the output's precision after the temperature of the sensor reaches the activation temperature in order to detect a decline in performance of the air/fuel ratio sensor, it is not always necessary to make that determination in order to detect a disconnection abnormality. Notwithstanding, the abnormality detecting apparatus for the air/fuel ratio sensor is unable to detect a disconnection abnormality until the temperature of the sensor reaches the activation temperature. As a result, a disconnection abnormality cannot be detected early on, so that measures in response to that abnormality cannot be taken at an early stage.